Plasma ion technology is commonly used for semiconductor wafer processing. Ion sources designed to process an entire semiconductor wafer are required to provide a uniform beam of ions over a broad area, such as over a 300 mm wafer. One application of such plasma systems would be to remove photoresist. The plasma produces a reactive species, such as monatomic fluorine or oxygen, that reacts with the photoresist to turn it to ash, which is subsequently removed by the vacuum pump. Because the ashing is performed by a reactive species, rather than by sputtering, the energy of the ions impacting the work piece can be relatively low and the voltage difference between the interior of the plasma chamber and the work piece is generally low.
The requirements for plasma sources used in focused beam systems, such as a scanning electron microscope or a focused ion beam system, are significantly different requirements from those used in whole wafer processing systems. Ion or electron beams are focused to sub-micron spots. Because the focusing system typically projects a demagnified image of the virtual ion source onto the sample, the virtual ion source from which the charged particles are emitted should be small to produce a small spot on the sample. That is, the ion source should provide a relatively large number of ions coming from a small area. The energies of all the charged particles in the beam must be similar, or chromatic aberration will prevent the charged particles from focusing to a fine point.
Inductively coupled (IC) plasma sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. The inductively coupled plasma source is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a small spot. IC plasma sources, such as the one described in U.S. Pat. No. 7,241,361, to Keller et al. for a “Magnetically Enhanced, Inductively Coupled Plasma Source for a Focused Ion Beam System,” which is assigned to the assignee of the present invention and is here by incorporated by reference, include a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the gas in an ionized state within the chamber.
Focused charged particle beam systems are often used to etch or deposit material or to form an image of a work piece and the charged particles are accelerated to relatively high energies by a voltage difference between the source and the work piece. Ions in a focused ion beam typically impact the work piece with landing energies of between 5 keV and 100 keV, more typically at about 30 keV. Electron landing energies vary between about 500 eV to 5 keV for a scanning electron microscope system to several hundred thousand electron volts for a transmission electron microscope system. To reduce risk to personnel, the work piece is typically maintained at or near ground potential, and the plasma source is maintained at a high voltage, either positive or negative, depending on the particles used to form the beam. “High voltage” as used herein means positive or negative voltage greater than about 500 V above or below ground potential. For the safety of operating personnel, it is necessary to electrically isolate the high voltage components. The electrical isolation of the high voltage plasma creates several design problems that are difficult to solve in light of other goals for a plasma source design.
For example, it is desirable to place the radio frequency coils that provide power to the plasma as close as possible to the plasma to efficiently transfer power, but having a difference in voltage across a small distance leads to arcing, which can damage the system. As described above, the plasma is maintained at a high DC voltage, while the radio frequency coils are typically oscillated at an RF voltage that is high, but lower than the DC voltage of the plasma. One way to eliminate much of the voltage difference between the plasma and the coil would be to maintaining the coils at a DC bias equal to the high potential as the plasma. This solution, however, would require maintaining the power supply for the coil at the high plasma potential, which would excessively complicate the power supply design and greatly increase the cost.
FIG. 1 shows schematically a typical prior art plasma source 100 for use in a focused ion beam system. The plasma source 100 includes a plasma chamber 102. Plasma 104 is maintained within the plasma chamber 102. A plasma extraction and biasing electrode 106 encloses the bottom portion of plasma chamber 102. Electrode 106 has an exterior high voltage connection to a bias voltage source 107 that is used to bias the plasma to its target potential, typically to 30 kV for most charged particle beam applications. Electrons or ions are directed from the plasma source through opening 112 to a work piece. A split Faraday shield 108 is placed between plasma chamber 102 and RF coils 110. Shield 108 is typically well grounded, and cooling fluids can be delivered in the space between plasma chamber 102 and RF coils 110. For the purpose of forming small probes with minimal chromatic aberrations, a Faraday shield can help prevent RF induced energy spread. Faraday shield for plasma ion sources are described, for example, in Johnson, Wayne L., “Electrostatically-Shielded Inductively-Coupled RF Plasma Sources,” HIGH DENSITY PLASMA SOURCES: DESIGN AND PHYSICS AND PERFORMANCE, Popov, Oleg, A., Ed. (1995).
The grounded Faraday shield isolates the high DC voltage plasma from the radio frequency coil potential and reduces capacitive coupling between the coil and the plasma. When the Faraday shield is located close to the dielectric plasma container, the large electric field caused by the voltage difference between the grounded shield and the plasma being biased to the necessary accelerating voltage (e.g. 30 kV) causes the possibility of a high voltage discharge which could damage the source. The presence of air trapped between the Faraday shield and the ceramic also causes the potential for arcing in that region.
The introduction of the Faraday shield between the plasma chamber and the antenna also inevitably leads to the antenna being placed further away from the plasma vessel, which can cause complications including arcing from the antenna to the shield and from the shield to the plasma. Furthermore, Faraday shields may have sharp edges which cause additional high voltage management concerns.
Not only is high voltage management a design challenge for a plasma source for a focused ion beam system, heat management is also a challenge. The energy applied to the plasma chamber generates heat in the plasma chamber and in the radio frequency coils. While a compact plasma source is desirable for beam formation, the more compact and powerful the plasma source, the hotter the source becomes and therefore the greater the need to efficiently dissipate the heat. The high voltage can also make cooling difficult, which can limit the density of the plasma used. These conflicting requirements make the design of an ICP source very challenging. U.S. patent application Ser. No. 13/182,925, which is assigned to the assignee of the present invention, describes a liquid cooled plasma source for a focused charged particle beam system.
The Faraday shield can complicate the cooling methods. Fluids in contact with the shield must not erode or oxidize the shield, and the choice of cooling fluid may also be limited by the dielectric concerns associated with the high voltage management at the sharp edges of the Faraday shield.
M. Dickson, et al., “Radial Uniformity of an External-Coil Ionized Physical Vapor Deposition Source,” J. Vac. Sci. Tech. B 16(2) (1998), describes a plasma system for ionized physical vapor deposition of metal onto wafers. The plasma ions bombard a metal target, and the metal sputtered from the target are ionized and directed toward the wafer below by the plasma sheath. Because the sputtered metal from the target will quickly form an electrical path around the inside of the quartz chamber, a water-cooled metal Faraday shield is used inside the plasma chamber. The Faraday shield within the plasma chamber acts as a baffle for sputtered metal and protects the dielectric walls of the plasma chamber from thick deposits of metal film. The water-cooling lines are electrically isolated from ground and connected through isolated radio frequency power feedthroughs. The large, wafer scale plasma chamber of M. Dickson, et al., facilitates the engineering of the Faraday shield within the large plasma chamber.